To enable the high data rates required for next generation wireless systems, orthogonal frequency division multiple access (OFDMA) technology has become popular. In an OFDMA network, each macrocell mobile station (mMS) is synchronized with a macrocell base station (mBS) such that the mobile stations stagger their transmission times so that all uplink symbols arrive at the macrocell base station simultaneously. To provide this synchronized transmission, the mobile stations time their transmissions according to their range from the base station. The mobile stations at the outer edge of the macrocell start their transmission first whereas mobile stations closer to the base station will transmit in a delayed fashion with regard to the outlying mobile stations. In this fashion, the transmissions from the variously-distributed mobile stations arrive simultaneously at the base station.
This staggered transmission by mobile stations in an OFDMA (or OFDM) network presents challenges to the incorporation of femtocell networks within the macrocellular network. In that regard, femtocell networks are one of the promising technologies for next generation wireless communication systems. They satisfy the demand for higher data rates, reduce the costs of service providers, enable better in-door coverage, and reduce the load on macrocellular networks. But the macrocell mobile stations do not synchronize their transmissions with regard to a femtocell base station (fBS) but instead to the macrocell base station as discussed above. Thus, whereas the macrocell mobile stations' uplink transmissions are received synchronously at the macrocell base station, these same transmissions will arrive asynchronously at the femtocell base station. The femtocell network may operate on a dedicated channel to avoid co-channel interference from the macrocell data traffic. But the difference between the arrival times of the uplink signals at the femtocell base station can introduce inter-carrier-interference (ICI) with respect to OFDMA signaling parameters such as the cyclic prefix (CP) length. The sub-carriers carrying the macrocell traffic will thus interfere with the femtocell sub-carriers even though the femtocell network operates on a dedicated channel.
The statistics for this asynchronous reception at the femtocell base station depends on a location of the femtocell base station within the macrocell and the distribution of the macrocell mobile stations within the macrocell. FIG. 1 illustrates an example femtocell-containing macrocell scenario. A macrocell mobile station 120 is relatively close to a femtocell base station 150 whereas a macrocell mobile station 160 is relatively farther away from femtocell base station 150. Femtocell base station 150 may be assumed to be synchronized to the first arriving mobile station uplink signal, which in FIG. 1 would correspond to the uplink transmission from mobile station 120. If the remaining mobile stations are arrayed in the macrocell relatively close to mobile station 120, the arrival times for the uplink transmissions from these remaining mobile stations may be captured within the CP length of the femtocell signal, thereby limiting inter-carrier interference (ICI) at the femtocell base station. However, if the femtocell base station is deployed near the macrocell edge, variance of the macrocell mobile station uplink signal arrival times at the femtocell base station will be larger depending on the location of the macrocell mobile stations within the macrocell. For example, the range between macrocell mobile station 160 and the femtocell base station may be such that the uplink signals from station 160 arrive at the femtocell base station with a delay that exceeds the CP length, which causes ICI at the femtocell base station.
Accordingly, there is a need in the art for ICI mitigation techniques for dedicated channel femtocell networks.